Rechargeable electric storage batteries, for example lead-acid batteries are used in industrial equipment such as lift trucks. In such use it is important to establish a battery maintenance strategy that provides for the most efficient use of the battery and the battery charger. A battery should be so sized and charged sufficiently that it will be ready for recharging at the end of a work period, for example a shift, with the discharge level being appropriate to satisfy the charger requirements. The battery maintenance strategy should also involve the use of an appropriate battery monitoring technique to ensure that charged batteries are not unnecessarily put on charge and to ensure the batteries are not discharged beyond an acceptable level.
With a lead-acid battery, charging an already charged battery, say with a specific gravity (SG) above 1.240, increases water consumption, damages the battery plates and removes up to one cycle from the potential life of the battery. On the other hand, battery manufacturers advise that discharging a battery below an SG of about 1.140 (80% of useable capacity) will damage the battery and cancel its warranty. In general terms, for every 10 points of SG maintained above the manufacturer's maximum discharge level, an extra year of battery life may be realized. Another factor to consider is the charger requirements, which generally dictate a minimum charge of SG 1.160 to 1.185. The optimum level for maximum battery discharge is considered to between SG 1.170 and 1.150. It is therefore an important part of a battery maintenance program to have an effective battery charge monitoring method and apparatus.